In an attempt to meet licensing requirements of the Nuclear Regulatory Commission, NRC, (formerly the U.S.A.E.C.), fuel designers have had to increase the number of rods per assembly, thereby reducing the LHGR (linear heat generation rates). Along with this change a small size fuel rod is required and a larger number of control rod guide tubes per assembly e.g., a switch from 15 .times. 15 to a 17 .times. 17 fuel array keeping the same external dimensions, with the number of control rod guide tubes increasing from 16 per assembly to 20 or 24. The number and location of control rod guide tubes in the lower LHGR design is not compatible with the number and location of the control rod guide tubes of the higher LHGR design now in use. This change in fuel assembly design would normally require a redesign and retrofitting of the control rod clusters as well as the control rod guides in the upper core structure which is separate from the control rods or the fuel, upon switching from the 15 .times. 15 design to the 17 .times. 17 design in an existing reactor. This retrofitting is costly and entails repair of a radioactive upper structure measuring approximately 15 ft. in diameter. Such repair must be performed at the reactor site and is expected to lead to reactor shutdown over periods of several weeks.
The subject invention is intended to enable continued use of existing control rods and control rod guide structures while reducing both maximum and average linear heat generation rates of a bundle as required by licensing criteria (10CFR50, Appendix K).
Prior art patents, specifically considered in connection with the present invention include the following U.S. Pat. Nos.: 2,848,404 -- Treshow; 2,961,393 -- Monson; 3,036,964 -- Horning; 3,049,487 -- Harrer et al; 3,060,111 -- Sherman et al; 3,206,369 -- Fortesque et al; 3,211,621 -- Kreegan; 3,147,191 -- Crowder; 3,733,252 -- Georges et al and 3,745,069 -- Sofer et al.
While these patents disclose the use of fuel rods of the same size containing different types of nuclear fuel, none disclose an arrangement wherein there is provided a hybrid fuel assembly having different size fuel rods, and more particularly wherein a basic fuel array with islands of fuel rod clusters is used.